The concept of harmony was both intrinsic to the music of the Everly Brothers and sometimes foreign to their personal relationship.

Nevertheless, Phil, who died Friday at age 74, and Don, 76, forged an endearing sound and a half-century career that saw them hit the Billboard top 40 chart 26 times, influence artists ranging from The Beatles and the Beach Boys to Linda Ronstadt, Simon & Garfunkel, Neil Young and Alison Krauss, and land in both the Rock and Roll Hall of Fame and the Country Music Hall of Fame.

Don usually sang the baritone notes and most of the lead parts while Phil handled the higher range, their voices intertwining organically, almost supernaturally, on classics such as Cathy's Clown,Wake Up Little Susie, Crying in the Rain, Bye Bye Love, All I Have to Do Is Dream, Walk Right Back and Claudette.

Phil was born in Chicago into a family of traveling musicians and lived in Iowa as a child, but the brother act he formed with Kentucky-born Don in the mid-1950s drew its inspiration from the sounds of the Appalachian Mountains.

Country and bluegrass acts such as the Delmore Brothers, Osborne Brothers and Louvin Brothers formed a template for the close-harmony sound, and the guitar-playing Everlys took it in a smoother, pop-rock direction with material written by themselves and Nashville tunesmiths, most notably Felice and Boudleaux Bryant. The influence of Texan Buddy Holly, with whom they toured in the late 1950s, also can be heard in their sound (and Phil would serve as a pallbearer at Holly's funeral following his fatal plane crash in 1959).

Behind the scenes, their partnership was sometimes problematic — they battled drugs, managers and each other. At a California concert in 1973, their feuding erupted onstage and they split, barely speaking for a decade, during which time they pursued solo careers with minor success.

A reunion that began in 1983 resulted in a well-received live album and two excellent studio albums, EB '84 (which featured the Paul McCartney-written hit On the Wings of a Nightingale) and Born Yesterday.

Neil Young inducted the brothers into the Rock Hall in 1986, where they were among the inaugural class of 10 performers. The duo released their final studio album, Some Hearts, in 1988 and continued to perform and collaborate with others into the late 1990s.

Two months before Phil died in Burbank, Calif., from complications of chronic obstructive pulmonary disease after a lifetime of smoking, Green Day's Billie Joe Armstrong and jazz-pop singer Norah Jones released an Everlys tribute album, Foreverly. Armstrong was full of admiration for the brothers.

The Everly harmonies "are so immaculate," Armstrong told USA TODAY. "And that record (the duo's second album, Songs Our Daddy Taught Us) was pretty daring at the time. A lot of other rock guys were trying to go pop. Chuck Berry had a string of big hits, and the same with Jerry Lee Lewis and Elvis. And here the Everlys were playing these torch songs and murder ballads. For them to do something so dark and angelic was appealing to me."

After Phil's death, Jones said in a statement: "The high harmonies Phil sang were fluid and so beautiful and always sound effortless in a way that just washes over the listener. He was one of our greats and it's very sad to lose him."

On Twitter, Boomers mourned, or at least Boomers with Twitter accounts: Presidential historian Michael Beschloss tweeted a concert poster from 50 years ago in England where the Everly Brothers got top billing over the Rolling Stones.